Electronic packages, such as those which use semi-conductor devices (chips), are used in increasing numbers in a wide variety of products. Generally, such electronic packages produce heat during operation. In some devices, the heat generated by the package's chip may build in the package and damage the package. Accordingly, there is often a need to dissipate the heat generated by electronic packages to extend the life of such packages.
Various ways of dissipating heat generated from electronic packages have been developed. One common method is to provide a fan in the device or adjacent thereto to blow air through the device and to vent the heated air. Unfortunately, a fan cannot always be provided because such packages are typically very small and/or located in a close environment with other components (e.g., on a circuit board). Another way to increase heat dissipation from an electronic device involves increasing the surface area of the device. According to well recognized heat transfer principles, increasing the surface area will cause an increased transfer of heat from the electronic component to its surroundings. As stated, however, it is generally desirable that electronic components be as small as possible. Yet another way to dissipate heat is to bond an electronic component to a heat sink. The purpose of the heat sink is to conduct heat away from the electronic package insulative cover material and then dissipate the heat from the heat sink. Heat sinks are typically made from a heat conductive material, such as metal, and aluminum is frequently used since it is lightweight and readily available. Such heat sinks can easily be designed to conduct heat away from the electrical component while simultaneously providing a large surface area for heat dissipation. For example, the heat sink may include integral fins that provide a large surface area for a relatively small volume.
While heat sinks solve some of the problems of heat dissipation, there remains a current problem of bonding a heat sink to an electronic component, as discussed in the aforementioned application Ser. No. 09/757,185, which is incorporated herein by reference. For the heat generated through the package's cover (or overmold) member to be transferred to the heat sink, a connection allowing heat conductance between the cover and the heat sink must exist. Conventionally, heat sinks are mechanically attached to electronic components and a heat conducting thermal grease is placed between the cover and the heat sink, thus providing heat conductance as needed. Unfortunately, mechanical attachment possesses serious disadvantages. First, such mechanical attachments require thermal grease, clips, and/or rivets, and other materials, increasing the material cost of the final package. Second, the mechanical attachments increase the process costs, and third, some mechanical attachments have proven to possess poor long term reliability. In other words, either the conductive path between the cover and heat sink is compromised or the mechanical attachment fails altogether. In addition, devices such as rivets can put stress on the electronic package and may even cause failure of the package during thermal expansion and contraction.
One attempt at resolving the problem of mechanical attachment involves using adhesive to create a uniform bond between the package's cover (overmold) member and a heat sink. Unfortunately, many electronic packages are made of substances or are assembled with substances to which it is very difficult to adhere a heat sink. Typical adhesives that are expected to bond to polymer compounds will not effective bond a heat sink to polymer compounds typically used as package overmold materials. This problem has been encountered in the production of plastic ball grid array (PBGA) packages, in particular, PBGA packages with an overmold cover over the package's chip(s). Examples of such packages are defined in U.S. Pat. Nos. 6,206,997, 5,729,440, 5,726,079 and 5,655,703. All of these patents, assigned to the assignee of this invention, are incorporated herein by reference.
In the aforementioned Ser. No. 09/757,185, and in its parent U.S. Pat. No. 6,206,997, there is defined an improved process for attaching a heat sink to the package's overmold material. This process involves exposing the formed overmold to a plasma for a sufficient period so as to assure at least partial conversion of silicon-containing residue to silica. Silicon is a known and often preferred material to be used when forming the overmold member. Silica conversion on the overmold's surface enhances subsequent heat sink attachment using a desired adherent material, e.g., a silicone-based paste adhesive (it being believed that the silicon in the adhesive interacts with the formed silica on the overmold's surface to produce a strong bond). In this embodiment, a metal oxide filler, e.g., alumina, is preferably added to improve the adhesive's heat conductivity from the overmold to the heat sink positioned thereon.
As understood from the following, the present invention defines another improvement to a process for attaching heat sinks to overmold members of an electronic package, and particularly when a process similar to that in defined Ser. No. 09/757,185 is utilized. It is believed that such an improvement will constitute an advancement in the art of electronic packages.